1. Field of the Invention
The present invention relates to a liquid ejection head that ejects a solution, in which charged particles are dispersed in a solvent, by means of an electrostatic force and a method of producing the same.
2. Description of the Related Art
Nowadays, a thermal-type ink jet head that ejects an ink droplet by means of an expansive force of an air bubble generated in ink under heating and a piezoelectric type ink jet head that ejects an ink droplet by giving a pressure to ink using a piezoelectric element have been proposed. In the case of the thermal-type ink jet head, the ink is partially heated to 300° C. or higher, so that a problem arises in that a material for the ink is limited. Also, when using the piezoelectric type ink jet head, a problem occurs in that its construction is complicated and an increase in cost is inevitable. As an ink jet head that solves those problems, a liquid ejection head is proposed which ejects a solution, in which charged particles are dispersed, by means of an electrostatic force (see JP 10-76664 A, JP 10-138493 A, JP 11-105293 A, JP 10-230608 A, or JP 9-309208 A for instance).
FIGS. 23A and 23B are each a schematic structural diagram showing an example of a recording head of an image forming apparatus disclosed in JP 10-76664 A, with FIG. 23A being a schematic cross-sectional view of the recording head and FIG. 23B being a diagram where a protrusion plate 420 is viewed from the right side in FIG. 23A. In a recording head 400, one pair of support members 401 and 402 made of an insulative material and having an approximately rectangular plate shape are arranged so as to oppose each other. A gap between these support members 401 and 402 is set as a recording liquid supply path (ink supply path) 404 and a recording liquid outflow opening (ink outflow opening) 404a is obtained in end portions of the support members 401 and 402 on the upper side in this drawing, In the recording liquid supply path 404, a recording liquid 406 is allowed to flow in the upward direction in the drawing at a predetermined pressure and this liquid 406 flows out from the recording liquid outflow opening 404a. The recording liquid 406 is ink in which positively charged colorant particles 405 are dispersed. First electrodes 411 are formed on the inner surface of the recording liquid supply path 404 (one of both surfaces of each support member 401/402) so as to reach the recording liquid outflow opening 404a. A protrusion plate 420 that is an ink guide member having a protrusion 422 at its tip end is arranged in the recording liquid supply path 404 so that the protrusion 422 protrudes from the recording liquid outflow opening 404a. Also, second electrodes 412 are formed on the surfaces of the protrusion plate 420 in regions opposing the support members 401 and 402. Further, a counter electrode 430 is arranged on the upper side in the drawing, with this counter electrode 430 being grounded. Still further, a recording medium 408 is arranged on a surface of the counter electrode 430 opposing the protrusion 422.
In the recording head 400 having this construction, a part of the recording liquid 406 overflown through the recording liquid outflow opening 404a moves upwardly along the protrusion plate 420 in proximity to the recording liquid outflow opening 404a and a meniscus 440 is formed on a surface of the protrusion 422 by means of the supply pressure, surface tension, and the like of the recording liquid 406. On the other hand, the great majority of the recording liquid 406 overflown through the recording liquid outflow opening 404a flows along the support members 401 and 402 and returns to a recording liquid tank (not shown).
When a positive bias voltage is applied to the first electrode 411 and the second electrode 412 under a state where the meniscus 440 of the recording liquid is formed on the surface of the protrusion 422 in this manner, an electric field is formed between the first and second electrodes 411 and 412 and the counter electrode 430. The colorant particles 405 in the recording liquid 406 move upwardly in the recording liquid supply path 404 toward the tip end of the protrusion 422 by means of this electric field and gather in proximity to the tip end of the protrusion 422. When a voltage having a predetermined pulse width is superimposed on the bias voltage and is applied to the first electrode 411 and the second electrode 412 under this state, the electric field formed between the first and second electrodes 411 and 412 and the counter electrode 430 is strengthened and the colorant particles 405 in the meniscus 440 are pulled toward a counter electrode 430 side. In this manner, the recording liquid 406 containing the colorant particles 405 is ejected toward the counter electrode 430 as a droplet. In JP 10-76664 A, a droplet of the recording liquid 406 is ejected in this manner and the colorant particles 405 are caused to adhere onto the recording medium 408.
FIG. 24 is a conceptual diagram schematically showing an example of an outlined construction of an ink jet head of an ink jet recording apparatus disclosed in JP 10-138493 A. An ink jet head 500 shown in this drawing includes a head substrate 502, an ink guide 504, an insulative substrate 506, an ejection electrode 508, a counter electrode 510 supporting a recording medium P, a bias voltage supply 512, and a signal voltage supply 514. Note that in this drawing, only one individual electrode serving as an ejection means constituting the ink jet head disclosed in JP 10-138493 A is conceptually illustrated.
Here, the ink guide 504 is made of a resin flat plate having a predetermined thickness and including a convex tip end portion 504a, and is arranged on the head substrate 502. Also, in the insulative substrate 506, a through-hole 516 is established at a position corresponding to arrangement of the ink guide 504. The ink guide 504 passes through the through-hole 516 established in the insulative substrate 506 and its tip end portion 504a protrudes upwardly from the upper surface of the insulative substrate 506 in the drawing, that is, from a surface thereof on a recording medium P side. Also, the head substrate 502 and the insulative substrate 506 are arranged so as to be spaced apart from each other by a predetermined distance, and a flow path 518 of ink Q is formed between these substrates 502 and 506.
Further, the ejection electrode 508 is provided in a ring manner for each individual electrode on the upper surface of the insulative substrate 506 in the drawing to surround the periphery of the through-hole 516 established in the insulative substrate 506. The ejection electrode 508 is connected to the signal voltage supply 514 that generates a pulse signal corresponding to ejection data (ejection signal) such as image data or print data, and the signal voltage supply 514 is grounded through the bias voltage supply 512. Also, the counter electrode 510 is arranged at a position opposing the tip end portion 504a of the ink guide 504 and is grounded. Further, the recording medium P is arranged on the lower surface of the counter electrode 510 in the drawing, that is, on a surface thereof on an ink guide 504 side, and the counter electrode 510 functions as a platen of the recording medium P.
In the ink jet head 500 constructed in this manner, at the time of recording, ink containing a fine particle component charged to the same polarity as a voltage applied to the ejection electrode 508 is circulated by an ink circulation mechanism (not shown) in a predetermined direction (from the right to the left in the illustrated example) in the ink flow path 518, and a part of the ink Q in the ink flow path 518 is supplied to the tip end portion 504a of the ink guide 504 through the through-hole 516 in the insulative substrate 506 by a capillary phenomenon or the like.
Here, a predetermined high voltage (DC voltage of 1.5 kV, for instance) is constantly applied to the ejection electrode 508 by the bias voltage supply 512. Under this state, the strength of an electric field in proximity to the tip end portion 504a of the ink guide 504 is low and the ink Q supplied to the tip end portion 504a will not fly out from the tip end portion 504a of the ink guide 504. Under this state, however, a part of the ink Q in the ink flow path 518, in particular, the charged fine particle component further moves upwardly so as to exceed the upper surface of the insulative substrate 506 in the drawing by passing through the through-hole 516 in the insulative substrate 506 and gathers around the tip end portion 504a of the ink guide 504.
When a pulse voltage of DC 500 V or the like (ON-time; 0 V:OFF-time) is applied by the signal voltage supply 514 to the ejection electrode 508 biased to the high voltage (DC 1.5 kV) by the bias voltage supply 512, both of these high voltages are superimposed on each other and a voltage (2 kV, for instance) is applied to the ejection electrode 508. As a result, the ink Q, in particular, the charged fine particle component in the ink Q further moves upwardly along the ink guide 504 and gathers in the tip end portion 504a. Then, the ink Q gathered in the tip end portion 504a of the ink guide 504 in this manner and containing the charged fine particle component flies out from the tip end portion 504a by means of an electrostatic force, is attracted by the grounded counter electrode 510, and adheres onto the recording medium P. In this manner, a dot is formed by the charged fine particle component.
By forming dots of the charged fine particle component in this manner while relatively moving the ink jet head 500 and the recording medium P supported on the counter electrode 510, an image corresponding to image data is recorded on the recording medium P.
Also, JP 11-105293 A discloses an ink jet head where ink is caused to flow along a protrusion plate that is an ink guide member and a meniscus is formed at a protrusion of the protrusion plate. This protrusion plate is produced by molding an electrode base made of alumina and sharpening a tip end thereof through grinding.
Further, JP 10-230608 A discloses an ink jet head where an ink guide member having a sharp-pointed portion is set so as to protrude from a surface of an ink layer flowing in a direction approximately perpendicular to an ink droplet ejection direction, a guide groove for guiding the ink from the ink layer to a tip end of the sharp-pointed portion is formed in the ink guide member, and an ink droplet is ejected from the tip end of the ink guide member by utilizing an electrostatic force. This ink guide member is formed through molding of a plastic resin.
Also, JP 09-309208 A discloses an ink jet head where no ink guide member is provided and a meniscus having an approximately hemispherical shape is formed at an ink outflow opening by means of the pressure of ink flowing out from an ink supply path and the surface tension of the ink and an ink droplet is ejected by utilizing an electrostatic force.
In the case of such an ink jet head that ejects ink that is a recording liquid by means of an electrostatic force, in order to eject a small ink droplet, it is required to form a meniscus at a tip end of an ink guide member serving as an ink droplet ejection position as finely as possible. Also, in order to eject a droplet having a stabilized size and shape, it is required to maintain the shape of a meniscus as constant as possible. Further, in order to eject ink droplets having a stabilized shape and size at a high ejection frequency, it is required to speedily supply ink to an ink ejection position by an amount decreased by ink droplet ejection and to restore the shape of a meniscus to a pre-ink-ejection state immediately after the ink ejection. Also, in order to eject a liquid with high density and high definition uniformly, it is required to form a sharp-pointed portion at an end of an ink guide member serving as an ink droplet ejection position with high density and high definition.
However, in the case of the ink jet heads described in JP 10-7.6664 A and JP 11-105293 A where ink is caused to flow along a protrusion plate that is an ink guide member toward a sharp-pointed portion and a meniscus of the ink is formed at a tip end thereof, the meniscus greatly fluctuates due to fluctuations of an ink supply pressure. Therefore, there is a problem in that it is impossible to eject an ink droplet having a stabilized size with high position accuracy.
Also, for the method disclosed in JP 11-105293 A with which a protrusion plate that is an ink guide member is produced by sharpening a tip end of an alumina-made electrode base through grinding, there is a problem in that it is impossible to unlimitedly increase the accuracy of a sharp-pointed shape of the protrusion plate and the number of process steps is increased.
Further, in the ink jet head disclosed in JP 10-230608 A where a meniscus is formed at a tip end of a sharp-pointed portion serving as an ink droplet ejection position using ink that moves upwardly along an ink guide groove, the ink moves upwardly toward the sharp-pointed portion by utilizing a capillary phenomenon. Therefore, there is a problem in that a long time is taken by ink supply and it is impossible to successively eject ink droplets having a stabilized size and colorant component concentration at a high ejection frequency.
Also, with the conventional ink guide member production method based on molding of a plastic resin, there arises a problem in that at the time of pulling-out of a plastic resin from a mold, the plastic resin adheres to the mold and is broken, which makes it impossible to perform molding into a desired shape. Therefore, it is difficult to produce an ink guide member so as to be sharply pointed with high accuracy. Also, in this method, it is required to arrange multiple molded ink guide members on a substrate while increasing position accuracy. However, it is impossible to unlimitedly increase the arrangement/position accuracy of the ink guide members. Further, a large number of process steps are required for arrangement.
Also, in the ink jet head disclosed in JP 09-309208 A where an approximately hemispherical meniscus is formed at an ink outflow opening by means of the pressure of ink flowing out from the ink outflow opening and the surface tension of the ink without providing an ink guide member, it is required to reduce the size of the ink outflow opening in order to form a fine meniscus. However, it is impossible to reduce the size of the ink outflow opening from a certain size because it is required to prevent ink clogging. Also, the shape of the meniscus greatly fluctuates due to fluctuations of the pressure of the ink flowing out from the ink outflow opening. For these reasons, there is a problem in that it is impossible to eject a minute ink droplet with stability.